Several methods exist for diagnosing disease that involve measuring a physical property of a part of the body. A change in such a physical property can signal the presence of disease. For example, x-ray techniques measure tissue physical density, ultrasound measures acoustic density, and thermal sensing techniques measures differences in tissue heat generation and conduction. Other properties are electrical, such as the impedance of a body part that is related to the resistance that the body part offers to the flow of electrical current through it.
Values of electrical impedance of various body tissues are well known through studies on intact humans or from excised tissue made available following therapeutic surgical procedures. In addition, it is well documented that a decrease in electrical impedance occurs in tissue as it undergoes cancerous changes. This finding is consistent over many animal species and tissue types, including, for example human breast cancers.
There have been a number of reports of attempts to detect breast tumors using electrical impedance imaging, such as, for example, U.S. Pat. No. 4,486,835. However, there are basic problems when trying to construct an image from impedance data. Electric current does not proceed in straight lines or in a single plane; it follows the path of least resistance, which is inevitably irregular and three-dimensional. As a result, the mathematics for constructing the impedance is very complex and requires simplifying assumptions that greatly decrease image fidelity and resolution.
Despite such difficulties, a method that permits comparisons of electrical properties for diagnostic purposes has been developed that involves homologous body parts, i.e., body parts that are substantially similar, such as a left breast and a right breast. In this method, the impedance of a body part of a patient is compared to the impedance of the homologous body part of the same patient. One technique for screening and diagnosing diseased states within the body using electrical impedance is disclosed in U.S. Pat. No. 6,122,544, which is incorporated herein by reference. In this patent, data are obtained from two anatomically homologous body regions, one of which may be affected by disease. Differences in the electrical properties of the two homologous body parts could signal disease. One subset of the data so obtained is processed and analyzed by structuring the data values as elements of an n×n impedance matrix. The matrices can be further characterized by their eigenvalues and eigenvectors. These matrices and/or their eigenvalues and eigenvectors can be subjected to a pattern recognition process to match for known normal or disease matrix or eigenvalue and eigenvectors patterns. The matrices and/or their eigenvalues and eigenvectors derived from each homologous body region can also be compared, respectively, to each other using various analytical methods and then subjected to criteria established for differentiating normal from diseased states.
Published international patent application, PCT/CA01/01788, which is incorporated herein by reference, discloses a breast electrode array for diagnosing the presence of a disease state in a living organism, wherein the electrode array comprises a flexible body, a plurality of flexible arms extending from the body, and a plurality of electrodes provided by the plurality of flexible arms, wherein the electrodes are arranged on the arms to obtain impedance measurements between respective electrodes. In one embodiment, the plurality of flexible arms are spaced around the flexible body and are provided with an electrode pair. In operation, the electrodes are selected so that the impedance data obtained will include elements of an n×n impedance matrix, plus other impedance values that are typically obtained with tetrapolar impedance measurements. Tetrapolar impedance measurements are associated with injecting current between so called current electrodes and measuring a voltage drop between associated electrodes. In a preferred embodiment, the differences between corresponding homologous impedance measurements in the two body parts are compared in a variety of ways that allow the calculation of metrics that can serve to either indicate the presence of disease or localize the disease to a specific breast quadrant or sector. The impedance differences are also displayed graphically, for example in a frontal plane representation of the breast by partitioning the impedance differences into pixel elements throughout the plane.
Despite the attractive features of this method of diagnosing disease in one of a homologous pair of body parts, there are some problems associated with this straightforward implementation. In particular, the current path through the body part, whether healthy or not, as the current flows from one electrode to the other is, in general, complex. It encompasses to a certain extent, all areas of the body part. In the aforementioned method, this complexity is addressed by simplifying assumptions. This simplification may affect the ability of the method to detect the disease.